Ultrastructural characterization of porcine oocytes and adjacent follicular cells during follicle development: lipid component evolution

The objective of this study was to characterize the morphometry and ultrastructure of porcine preantral and antral follicles, especially the lipid component evolution. Ovarian tissue was processed for light microscopy. Ovarian tissue and dissected antral follicles (< 2, 2-4, and 4-6 mm) were also processed for transmission electron microscopy using routine methods and using an osmium-imidazole method for lipid detection. Primordial follicles (34 ± 5 μm in diameter, mean ± SD) had one layer of flattened-cuboidal granulosa cells around the oocyte, primary follicles (40 ± 7 μm) had a single layer of cuboidal granulosa cells around the oocyte, and secondary follicles (102 ± 58 μm) had two or more layers of cuboidal granulosa cells around the oocyte. Preantral follicle oocytes had many round mitochondria and both rough and smooth endoplasmic reticulum. In oocytes of primordial and primary follicles, lipid droplets were abundant and were mostly located at the cell poles. In secondary and antral follicles, the zona pellucida completely surrounded the oocyte, whereas some microvilli and granulosa cells projected through it. Numerous electron-lucent vesicles and vacuoles were present in the oolemma of secondary and antral follicles. Based on osmium-imidazole staining, most of these structures were shown to be lipid droplets. As the follicle developed, the appearance of the lipid droplets changed from small and black to large and gray, dark or dark with light streaks, suggesting that their nature may change over time. In summary, although porcine follicles and oocytes had many similarities to those of other mammalian species, they were rich in lipids, with lipid droplets with varying morphological patterns as the follicle developed.